How to: Clean LIGO
To search for ripples in space and time, the Laser Interferometer Gravitational-wave Observatory (LIGO) uses a laser beam that is split in two and travels down perpendicular 2.5-mile arms containing mirrors at their far ends. The beam reflects off the mirrors and bounces back to converge where the arms meet.
A passing gravitational wave will stretch and squeeze space itself, causing the distance a light beam travels to increase or decrease ever so slightly; this changes the way the split beams ultimately converge. So far, so (relatively) straightforward.
The tricky part? The setup for LIGO—at its two facilities, one in Washington and one in Louisiana—comprises tens of thousands of pieces of equipment, from massive optics to tiny screws. And if anything has so much as a speck of dust on it, it might contaminate LIGO’s optics and diminish the signal of a gravitational wave. Here are some of the things Caltech engineers do to make sure that everything is beyond squeaky clean.
1. To clean LIGO’s unique optics, a special polymer blend solution is applied. This technique cleans the surfaces without leaving the tiny scratches associated with the more traditional drag-wipe technique.
2. After the polymer dries, it is peeled from the surface of the optic.
3. A spray of neutralized ions prevents static buildup, keeping dust from being attracted to the freshly cleaned optic.
4. The mirrors are inspected using an extremely bright light kept within a black enclosure, to minimize light from other sources.
5. Final cleaning and inspection takes place in a custom-built clean room.
6. Smaller items like screws and bolts are placed in an ultrasonic bath for cleaning.
7. Post bath, the instruments spend time in a high-temperature vacuum oven that reaches 200 degrees Celsius (upwards of 390 degrees Fahrenheit).
An engineer prepares a light baffle that will capture laser light scattered by dust and imperfect optics, preventing the stray light from contaminating the gravitational-wave signal. Photos: Caltech/MIT/UGO Lab